1. Field of the Invention
This invention generally relates to an apparatus for controlling a throttle valve, and specifically relates to a throttle valve control apparatus including a guard mechanism for mechanically limiting a maximum degree of opening of a throttle valve.
2. Description of the Prior Art
Some of advanced automotive vehicles have a link-less throttle system in which the degree of depression of an accelerator pedal is converted into an electric signal, and a throttle valve is driven by an actuator in response to the electric signal.
The link-less throttle system of a given type is equipped with a guard mechanism for mechanically limiting the maximum degree of opening of the throttle valve. Specifically, the guard mechanism is mechanically connected to the accelerator pedal. To cope with a failure of a control system between the accelerator pedal and the throttle valve which might cause wrong positioning of the throttle valve, the guard mechanism prevents the actual degree of opening of the throttle valve from exceeding a target degree which is determined by the degree of depression of the accelerator pedal. Normally, the guard mechanism allows the actual degree of opening of the throttle valve to follow the target degree.
Automotive cruise control or automotive engine idle speed control requires movement of a throttle valve independent of the position of an accelerator pedal. Thus, the link-less throttle system of the above-mentioned type is generally unsuited for use in automotive cruise control or automotive engine idle speed control.
Japanese published unexamined patent application 63-201335 discloses an advanced link-less throttle system which can be used in automotive cruise control. In the link-less throttle system of Japanese application 63-201335, when system conditions are normal, the degree of opening of a throttle valve which is determined by a guard mechanism is held by a vacuum responsive actuator at a degree corresponding to the maximum degree of opening of the throttle valve (that is, the fully-open position of the throttle valve). Thus, in this case, the guard mechanism is disabled and therefore the throttle valve can be moved independent of the degree of depression of an accelerator pedal so that automotive cruise control can be executed. On the other hand, when a system failure is detected, the vacuum responsive actuator moves the guard mechanism into engagement with the throttle valve to urge the throttle valve toward its closed position. At the same time, the vacuum responsive actuator moves the guard mechanism into engagement with the accelerator pedal, and hence the degree of depression of the accelerator pedal is mechanically transmitted to the throttle valve via the guard mechanism. In this case, the guard mechanism is enabled, and the throttle valve is moved in accordance with the degree of depression of the accelerator pedal.
In the link-less throttle system of Japanese application 63-201335, when a system failure is detected, the accelerator pedal is subjected to both the force of an accelerator pedal return spring and the force of the vacuum responsive actuator. On the other hand, under normal system conditions, the accelerator is free from the force of the vacuum responsive actuator. Thus, when the accelerator pedal is depressed to terminate cruise control and to accelerate a vehicle from a cruise control speed, only the force of the return spring counteracts the depression of the accelerator pedal so that the depression of the accelerator pedal is too easy. The easy depression of the accelerator pedal tends to cause excessive depression thereof which results in a reduction of a vehicle drivability or a vehicle fuel consumption rate.
In the link-less throttle system of Japanese application 63-201335, when a system failure is detected, it takes a certain length of time to move the guard mechanism from the disabled state into an operable (enabled) state. This length of time generally causes a reduction of system response characteristics.